The present invention relates to a projection display, and more particularly to a device for preventing white chromaticity deviation (color deviation) in a projection display which projects the respective images emitted from tubes of projecting monochromes such as red, green and blue on a rectangular screen through lenses arranged correspondingly to the projection tubes, thereby to compose the images by overlapping.
If in the projection display of this type, the light amounts projected from the respective monochrome projection tubes through the lenses are equal at an optional position (spot) of the screen, a white image will appear at the spot, but a color deviation means that a color other than white will appear at such spot.
Owing to relative positions where the monochrome projection tubes for red, green and blue are arranged, however, the light amounts coming from the respective monochrome projection tube are not actually equal at some position (spot). For example, if at the spot in issue, the amount of red light from the red projection tube is more than that from the other projection tube (green or blue), a reddish color which should be naturally white will appear at the spot. This means that "color deviation" (hereinafter referred to as white chromaticity deviation as the case may be) will occur. Likewise, if at the spot in issue, the amount of blue light from the blue projection tube is more than that from the other projection tube (green or red), a bluish color which should be naturally white will appear at the position. This also results in occurrence of "color deviation" (white chromaticity deviation). Such a phenomenon is that although white images should uniformly appear on a screen, reddish or bluish images appear at some position (spot); it, therefore, provides poor color rendition and hence deteriorates the resultant image quality.
The present invention is directed to a projection display can decrease such white chromaticity deviation and so enhance color fidelity.
An optical system of the projection display disclosed in JP-A-61-29189 is shown in FIG. 2 in its horizontal section. In FIG. 2, items 1, 2 and 3 are projection tubes corresponding to red, green and blue, respectively. Items 4, 5 and 6 are projection lenses corresponding to the tubes 1, 2 and 3, respectively. Item 7 is a screen. The respective projection tubes are arranged in parallel with each other and also the respectivee lenses are arranged in parallel with each other. Such an arrangement is advantageous in that color misregistration does not occur as long as geometrical distortion of the lenses is negligibly small.
The field angle by which the green lens 5 sees the screen 7 is symmetrical with respect to its optical axis as indicated for the right and left ends of the screen in FIG. 2. On the other hand, the field angles at which each of the red and blue projection lenses 4 and 6 sees the screen 7 are asymmetrical for the ends of the screen as indicated as (large and small) since the direction of the light advancing from the pupil of the lens to the center of the screen 7 deviates by a convergence angle positively or negatively from that in the green projection lens 5.
The light transmission of a projection lens generally deteriorates according as its field angle increases. In the optical system of FIG. 2, therefore, at the right end of the screen, the amount of light coming from the red lens 4 is more than that coming from the other lenses; as a result, although the spot in issue should be white as long as the light amount coming from any projection lens is equal, the spot deviates to be reddish. Likewise, at the left end of the screen, the spot which should be white deviates to be bluish.
In order to correct the white deviation that the spot to be white deviates to the other color, it is proposed in JP-A-61-29181 to previously modulate the amounts of electron beams in red and blue projection tubes in the horizontal direction with no necessity of modulating them in the vertical direction.
In the ordinary optical system in which deviation in the field angle indicated in FIG. 2 exceeds about 0.1 rad (radian), however, the amount to be corrected is too large and hence the residual deviation cannot be disregarded. Moreover, in such an optical system, the field angle of red or blue is too large at one of the right and left ends and hence the focusing performance of the lenses will deteriorate.